As described in the U.S. Pat. No. 4,165,616, obtaining highest possible stage efficiencies and avoiding negative reactions on all turbine blades require axial velocities to be maintained within a specific range. Axial velocity of steam exiting a rotatable turbine blade is one of the most significant parameters for determining stage loading, probability of negative reaction, and probability of a turbine stage doing negative work. Last stage or exhaust blades in a turbine are the most difficult blades to optimally design since they are exposed to widely varying pressure ratios due to part load and overload operations.
When exhaust pressures downstream from the exhaust stage vary, last stage blade optimization becomes even more difficult and often results in blades whose peak efficiencies may be rather low. Relatively small variations in exhaust pressure can have a substantial effect on turbine performance. The effect is especially pronounced when the turbine is operating at part load, during startup, or during shutdown where a change in back pressure for any given mass flow rate can cause the exhaust stage's mode of operation to change from zero work to choked flow or vice versa. The normal operation point for turbines is usually designed to fall between the two aforementioned extremes. Operation in the choked flow region would yield no additional turbine power output, but would increase the heat rate of the cycle whereas operation beyond the zero work region would cause consumption of, rather than production of, work generated by the remainder of the turbine blades.
An additional disadvantage to operating beyond the zero work point is that the last stage would eventually experience the unsteady flow phenomenon which can cause extraordinarily large blade vibrations. An additional reason for avoiding operation beyond the choke point is the discontinuous flow patterns which result upstream and downstream from the choke point. Such discontinuous and unsteady flow adds vectorially to any stimulating vibratory force on the blade caused by external forces.
It is generally known to provide shrouds at the tip and/or snubbers at a mid-height point to rotating blades to prevent vibration. The U.S. Pat. No. 3,751,182 describes a form of guide vanes fastened to adjacent rotating blades near the tip of the blades to connect the blades such as to reduce vibrations.
In view of the prior art it is seen as an object of the present invention to provide an arrangement of static vanes, in particular of the static vanes in the last stage blades of a low pressure steam turbine. The arrangement is preferably designed to reduce blade vibrations.